Our limited understanding of the structural basis for RNA function exists in part because relatively little is known about the three-dimensional structures of RNAs. Lingand binding RNAs (aptamers) isolated from random sequence pools by in vitro selection are ideal model systems for RNA structure-function studies. We have succeeded in growing diffracting crystals of liganded complexes formed by two such RNAs, selected to bind either cyanocobalamin (vitamin B12) or biotin (vitamin H). We have obtained partial phase information from MAD experiments on the cyanocobalamin binding aptamer and are currently refining heavy atom sites to improve phasing. We have used diffraction obtained from the biotin binding aptamer to improve crystallization and freezing conditions and are currently screening for derivatives.